In recent years, organic light-emitting diodes (OLEDs) have become more and more important. Besides applications in the field of displays, applications for lighting purposes also have increasingly moved to the centre of development work. The big potential of the technology in this field has been recognized generically and it is assumed that OLEDs are going to become one of the most important technologies in the field of lamps in future. Meanwhile, both the performance efficiencies and the life of components have reached a competitive level in comparison to alternative lighting technologies, such as incandescent lamps, fluorescent lamps or inorganic LEDs.
Nevertheless, some technical obstacles which hitherto stand in the way of a commercial breakthrough in the market have not yet been overcome. One of these obstacles is to increase the total quantity of light a component can emit. This quantity of light is substantially determined by two factors, on the one hand the surface area of the OLED and on the other hand its light intensity. The latter can not be increased at will as increasing light intensity brings about an exponential reduction of the life. Furthermore, the area adjacent between the electrodes of the OLED will become that big with a certain light intensity that it leads to a breakdown and thus the formation of a short (circuit). To date, it is assumed that the application light intensities will be in the range of from 500 to 10,000 cd/m2.
Thus, enlarging the active lighting area primarily remains to increase the quantity of light. For future OLED applications in the lighting sector, the active areas are thus presumably going to lie within a range of a few square centimetres up to a square meter and more. As OLEDs typically are operated at a voltage of about 2-10 V and this voltage is independent of the area, the flowing current increases with increasing total area. For example, with a current efficiency of the component of 50 cd/A and an operating light intensity of 5000 cd/m2, a current of 1 ampere would already flow with a comparatively small area of 100 cm2.
Supplying such a high current would already cause hardly solvable problems in most practical applications. Another enlargement of the area would aggravate these problems further.
As a solution to this problem, it was already suggested to connect a plurality of small OLED elements with a comparatively small area in series (GB 2392023 A). Through this, the operating voltage of the component is increased by a factor which roughly corresponds to the number of OLED elements connected in series. At the same time, the operating current is likewise reduced by about this factor. Thus, the total quantity of light and the performance efficiency may be kept the same; however, the electrical actuation of the component is markedly simplified as it is usually considerably easier to supply a high operating voltage.
An advantageous characteristic of this serial connection of OLED elements within a component is that no interruption of the current path occurs in the case of the formation of a short between cathode and anode of an OLED element. Even if a part of the component, namely the OLED element with the short, no longer emits light in this case, the emitted total quantity of light of the component nevertheless remains virtually unchanged as a correspondingly higher voltage is now applied to the other, remaining elements. Thus, even after the formation of a short, such a component can still serve its purpose.
However, if a component solely consists of a single large OLED, the luminous power is reduced significantly after the formation of a short as now a large part of the current flows over the short circuit without being converted into light. If a constant distribution voltage is applied to the component, the flowing current would also dramatically increase which in many cases will result in a strong temperature rise at the point of the short. For these reasons, components having a single large OLED area can no longer be used after the formation of a short.
However, the production of OLED components with a serial connection of OLED elements is markedly more complex than the production of a single large diode.
Firstly, it is the structuring of the electrode on the substrate side is required for such a solution to define the sub-electrodes of the serial connection. Furthermore, the organic layers and the top electrode also have to be structured to construct the serial connection. In principle, such a structuring is possible by means of different methods. Provided that the organic materials of the OLED are processed by means of vacuum evaporation, such a structuring can take place by means of shadow masks. Processing of the organics by means of transfer methods in which the organic materials are transferred from a backing film by means of local heating by a laser beam onto the substrate is also possible. However, this method can only be employed for the organic layers, not for the top electrode which commonly consists of metals, such as aluminum, silver, calcium or magnesium, or transparent oxides, such as indium tin oxide (ITO). To structure these materials, deposition via shadow masks is commonly likewise reverted to.
However, all these mentioned structuring methods increase the complexity of the production process which also inevitably makes this more expensive. The resolutions achievable with shadow masks are furthermore limited so that the minimum distance between OLED elements of the serial connection is limited. Furthermore, using shadow masks becomes more elaborate when the structures on the substrates are becoming smaller. If structures in the range of several 100 μm are to be formed, a fine adjustment of the shadow mask towards the substrate is usually required which is time-consuming and costly and commonly is performed by means of microscopes. However, if the structures to be reproduced are enlarged to a certain dimension, as is the case with a size of the OLED elements of about 1 cm2, for example, the orientation of the shadow mask with regard to the substrate may be performed with less precision. Through this, a simple orientation is then possible, for example by means of positioning pins, which is markedly faster and cheaper than a fine adjustment within the micrometre range. The orientation of the shadow mask with regard to the substrate can take place by means of positioning pins on the substrate or the substrate holder.
The masks themselves also would have to be manufactured with less precision so that cheaper production methods such as laser cutting may be employed instead of cost-intensive lithography methods. By a mask orientation by means of positioning pins, the cycle time required for the processing of the substrates is also shortened.
An improvement of OLED lighting elements on the basis of simplified processing of larger OLED elements having a common organic layer structure on strip-shaped structured base contacts has already been proposed. In such a component architecture, only the base electrode is finely structured which may be performed by means of photolithography. As a structuring of the substrate by photolithography usually has to be performed anyway, this results in barely higher process costs. The advantage of the strip-shaped structuring of the base electrode arises from the fact that through this, several OLED strips are connected in parallel within an OLED element having a common organic layer and a common counter electrode. If the formation of a short occurs in such a structure, the current flow over the strips of the base electrode is limited by its electric resistance. Thus, not the entire OLED element is inoperable in the case of a short but only a sub-strip. However, the limitation of the current flow is determined by the exact position of the short on the strip. If the distance that the current has to cover over the electrode strip to the short with the counter electrode is short, the ohmic resistance is also low and a high current flows through the short and not through the intact strips of the OLED element connected in parallel. Through this, efficiency losses of the component can occur after a short has fanned. For this reason, it would be desirable to electrically isolate a strip damaged by a short from the other strips.
For OLED displays, an electric isolation of individual pixels from the actuation feeds has already been discussed (M. Kimura, Y. Kubo, Analysis, “Detection and Repair of Pixel Shorts in PM-OLEDs”, IDWIAD Proceedings 2005, page 605). In this article, the authors suggest in the case of a pixel with a short to burn through the feed electrode made of metal or a conductive oxide by means of a laser beam or a high current pulse. However, both the separation by means of a laser and the burning-through of the feed by means of a current pulse requires a certain active effort. If the feed is to be cut through by means of a current pulse, it is furthermore required that the OLED region to be separated can be driven individually. Furthermore, the electrical feeds have to allow for such a high current flow without sustaining damage themselves. In any case, the actuation has to be performed actively by means of a current pulse, for example by means of suitable self-analysis and control electronics and thus does not take place directly and immediately via the formation of a short in the OLED element itself.
In the case of OLED lighting elements, it is typically not possible to electrically isolate a particular region by means of a current pulse.
The use of a laser for the separation is also highly complex. In particular for components which are already employed by the user, this method would require sending it in for repair which would cause high costs.